This invention was made with Government support under Grant No. MDA972-94-1-0002 awarded by ARPA. The Government has certain rights in this invention.
The continuing development of exceedingly small or so-called micro-devices such as micro-optic elements and micro-machines is of great importance to optoelectronic interconnection technologies and the development of communications and control systems. Diffractive optical elements such as spherical, cylindrical, Fresnel lenses, aspherics and other micro-devices having rather precise three dimensional profiles or contours present certain problems with respect to volume production of these elements of an acceptable quality, in particular. The fabrication of large arrays of such elements covering large areas is very costly with regard to known methods of production.
One technique for mass production of diffractive optical elements involves fabricating a master element which itself is made by etching processes similar to those used in the fabrication of micro-electronic circuits and similar devices wherein a multi-masking process using binary masks is conducted. The fabrication of a master or individual elements using a multi-binary mask method can result in significant dimensional errors in the master and the fabricated element due to residual alignment errors between consecutive masking steps. Although diamond turning, for example, can be employed in producing a master element, the multi-binary mask technique is limited with symmetric elements, for example. Still further, diffractive optical elements can be produced by injection molding, embossing or casting. However, the materials used in these techniques have limited optical and environmental properties, and are, for example, operable to be transmissive to radiation only in the spectral range visible to the human eye.
Some of the disadvantages of prior systems including those mentioned above have been overcome with the development of so-called gray scale masks which avoid multiple processing steps by providing a single mask which contains all of the information necessary for generating multi-phase levels, i.e., the three-dimensional contours required in a diffractive optical element and the like. Photographic emulsions have been used to provide gray scale masks which can be generated using a laser writer or an optical imaging system, for example. However, the high resolution required of diffractive optical elements and other micro-elements is limited with this technique due to the limited resolution of the laser writer and the graininess of the image on the emulsion based mask. Moreover, photographic emulsions are not particularly durable and do not allow cleaning of the mask with water or mechanical scrubbing.
Other gray scale masking techniques, including the so-called half tone binary mask, are also limited due to the small holes in the mask which will also diffract light passing through the mask, further limiting the resolution of the desired diffractive optical element, for example.
One improvement in the production of gray scale masks for use in fabricating diffractive optical elements and other micro-elements has been realized with the provision of a gray scale mask wherein different thicknesses of a light-absorbing material, such as Inconel, coated on a glass plate mask element, for example, can provide for the fabrication of a gray level mask with high resolution and compatibility with substantially all wavelength ranges used in optical lithography. However, one disadvantage of this technique is the cost of the mask generation method wherein multiple direct write steps are required to provide the lift off process of the light-absorbing material for each discrete thickness desired. The tight thickness control necessary in the material evaporation step makes this technique somewhat economically infeasible for many applications.
The use of a gray scale mask fabrication method for producing large quantities or large arrays of diffractive optical elements and similar micro-elements requiring high resolution of three dimensional contours has several advantages. Gray scale masks require only a single exposure of a photoresist when fabricating the elements on a substrate using an etching process. Gray scale masks thus avoid the alignment errors resulting from processes requiring the use of multiple binary masks. Moreover, if a suitable gray scale mask material is provided, thermal expansion and contraction of the mask can also be avoided.
Accordingly, there has been a continuing need to develop an improved fabrication method for relatively large quantities of and large arrays of micro-elements, such as diffractive optical elements or other elements covering large areas, such as computer generated holograms. It is to these ends that the present invention has been developed.